1. Field of the Invention
The invention relates to an electrophotographic copying device and, more specifically, to an improvement over the charging and cleaning of the support surface, also known as photoconductor on which the latent image of an original is developed.
2. Prior Art
The following U.S. Patents are representative of the prior art: U.S. Pat. Nos. 3,647,293; 3,637,306; and 3,736,055.
Numerous prior art teachings in the field of electrophotographic copying teaches various methods and devices for preparing or charging the surface of a photoconductor so as to obtain a latent image from an original document. Prints are then transferred from the latent image on the surface of the photoconductor, to a transferring media.
To enable the development of the latent image on the photoconductor and the transferring of said latent image to a transferring media, several stations are arranged in proximity to and to cooperate with the photoconductor to perform certain functions. At the charging station, the photoconductor is charged to a selective polarity, be it positive or negative. The photoconductor then moves to the exposing or imaging station where a latent image is copied from the original document. Next, the electrostatic latent image is developed at a developer station to form a toned image on the photoconductor. The toned image is then transferred from the photoconductor to another media at the transferring station. To complete the cycle, the photoconductor is erased, precleaned, and cleaned and is then ready for another cycle.
Although the prior art electrophotographic devices function adequately for the intended purpose, several problems plague these systems.
Probably one of the most pressing problems is the fact that the charging, transferring and precleaning functions are all performed by separate coronas at separate stations. These systems are referred to as a three corona type system. With this type of prior art design, the cost of the electrophotographic device is relatively high, due to the individual cost of each corona. Since the general trend is to minimize the cost of electrophotographic devices without sacrificing efficiency or copy quality, any reduction in the number of component counts, in the prior art devices, will be welcome.
Another problem relating to the separate processing station is the fact that each of the separate coronas require a separate power supply. The aggregate cost of these power supplies further augments the overall cost of the unit. As such, any reduction in the number of power supplies will reduce the cost of the unit.
It is common knowledge that conventional electrophotographic devices may be either a single cycle process or a two cycle process. In the typical two cycle process the photoconductor is charged, imaged and developed during the first cycle while the image is transferred and the photoconductor is cleaned in the second cycle. For satisfactory operation, some of the stations which render necessary functions during the copying process are active during the first cycle, while others are inactive and vice versa. On account of the rapid speed at which the photoconductor accesses each of the stations, it is, therefore, necessary for high speed switching to occur at these stations. The conventional 60 cycle power supply which is used for supplying power to these stations cannot withstand high speed switching. With these drawbacks, it is clear that a more efficient device which utilizes a more efficient charging technique is needed.
Several attempts have been made to improve the prior art electrophotographic devices by solving some of the above identified problems. For example, attempts have been made to combine the charge and the transfer corona station. At first blush, this combination seems to be workable and logical since the function of both stations is to supply charges having a given polarity. However, the combination, instead of solving the above described problems, creates additional problems.
One of the additional problems stems from the fact that the combined charge transfer station is designed with a grid structure to enhance the charge operation. However, transferring media which is fed into the machine at the charge/transfer station for transferring the latent image from the photoconductor jams into the grid wires. This jam results in machine breakdown.
For proper operation, if the charge on the toned image is positive, a negative charge has to be deposited onto the transferring media so that the positively charged toner particles will be attracted. Of course, if the toner is negatively charged then the transferring media has to be charged positively. With the presence of the grid assembly in the combined charge/transfer station, the charge (negative or positive) cannot be uniformly distributed onto the transfer media. With an uneven distribution of charges, the quality of the final copy is less than satisfactory.